The present invention is concerned with diagnostic test strips and an improved method for reading them by means of a reflectance spectrometer.
Test strips for the analysis of components in a liquid such as human body fluid are well known. Typically such strips are made of an absorbant material in which there is absorbed a reagent system which responds to the presence of an analyte in the test fluid with a visually detectable signal such as a change in color. This change in color, which appears in one or more test field of the strip, can be the result of an enzymatic reaction in which a redox dye is oxidized or reduced to produce the colored response. Alternatively, the strip is made of a material through which the analyte and labeled antibodies specific therefor can flow to form analyte/labeled antibody conjugates which are captured in a specific detection zone of the strip to provide a detectable response representing the concentration of the analyte in the fluid test sample.
While the detectable response obtained using such strips can be observed visually to obtain a qualitative or semi-quantitative measure of the analyte in the test sample, greater quantitation and faster, more reliable handling of multiple test strips can be realized by instrumentally reading the developed strips. Such instrumental reading is usually accomplished by the use of a reflectance spectrometer which determines the intensity of the reflection from the test field surface. This sort of instrument determines the intensity of the reflected light in the developed strip by illuminating the strip with light at one angle (typically 90.degree.), detecting the reflected light at a different angle (typically 45.degree.) and selecting the measured color or wavelength range at either the source or the detector.
Since the spectrometer is programmed to take the reflectance reading at a particular point in time and the intensity of the visually detectable signal can vary with a change in ambient temperature, because the reaction rate and/or equilibrium are often temperature dependent, there is a need for some means by which temperature variations can be factored out of the assay.
The present invention involves the use of thermochromic liquid crystals in conjunction with the reading of test strips by spectrophotometric means to aid in correcting the readout of the spectrophotometer for variations in ambient temperature. The use of thermochromic liquid crystals (TLCs) in research and testing is becoming increasingly widespread particularly in the areas of flow visualization and heat transfer studies. The TLCs react to changes in temperature by changing color as their name implies. They typically have chiral (twisted) molecular structures and consist of optical mixtures of organic chemicals. The proper name for these materials is cholesteric or chiral nematic liquid crystals. The term cholesteric is historical and is derived from the fact that the first materials to show the characteristic properties and structure of thermochromic liquid crystals were esters of cholesterol. However, many optically active chemicals and mixtures thereof which are not related to cholesterol or other sterols also exhibit the cholesteric liquid crystal structure. TLC mixtures can be divided into 2 distinct types according to their chemical compositions. These types are cholesteric, i.e. formulations comprised entirely of cholesterol and other sterol related chemicals and chiral nematic, i.e. formulations comprised entirely of non-sterol based chemicals. A third category of TLCs arises from the fact that cholesteric and chiral nematic chemicals can be mixed together to provide formulations which exhibit a continuum of physical and chemical properties somewhere between those of their pure cholesteric and pure chiral nematic precursors.
TLCs exhibit colors by selectively reflecting incident white light. Conventional temperature sensitive mixtures turn from colorless (black against black background) to red at a given temperature and, as the temperature increases, pass through the other colors of the visible spectrum in sequence (orange, yellow, green, blue, violet) before turning colorless (black) again at yet higher temperatures. Since the color changes are reversible, the color sequence is reversed upon cooling. TLCs can be used in a number of different forms such as unsealed liquids which are essentially oils with the consistence at their working temperatures being between that of a thin oil and a viscous paste which are applied in thin uniform films, microencapsulated forms in which droplets of the TLC are surrounded by a continuous polymer coating or coated sheets in which a thin film of the liquid crystal is sandwiched between a transparent polymer sheet as substrate and a black absorbing background.